Is it unreasonable to expect mother in law to pay for own guests at wedding?

My FH and I are trying to cut back on the costs for our wedding and have decided to have a small wedding of only about 50 people including family. My future mother in law wants to invite some friends and we have already included two friends of hers (who are husband and wife) and their kids but she wants to invite at least three more couples. This would be ok by us if we were having a separate ceremony and reception because then they could just come to the ceremony but we aren't. Our ceremony and reception are at the same place and the reception immediately follows the ceremony. We are paying for the whole wedding ourselves and cannot afford extra people. We have told her this already but she keeps insisting and kicking up a stink about it. She wants me to send her the guest list so she can add who she wants and approve of the other guests. She is very capable of holding a grudge and is very used to getting her own way. My FH is starting to buckle under her constant pressure (like always) and says we can cut back on other things (like the photographer, his suit and so on) and we can cut some of our friends from the list. I don't want to have remove any of our friends so my mil can invite people I have never met and FH has only met once or twice. Would it be rude to say that she can invite her friends to the wedding if she pays for them herself? Any suggestions on what to do about this?